Field of the Invention
A wide variety of solar systems exist for collecting solar energy for heating fluids and for producing electricity. The invention described herein is for heating, not for producing electricity.
Some solar panels used for heating focus or concentrate the solar flux, while others do not. The typical solar panel used for heating fluids such as relatively low temperature water do not concentrate the solar flux, while those used to produce high temperature fluids such as steam typically concentrate the solar flux. The invention described herein is for low temperature heating of fluids and thus is in the category of non-concentrating solar collectors. Such a solar panel is called a flat plate collector.
A typical flat plate solar panel for heating a fluid is shown in FIG. 1. Such solar panels typically include an insulated enclosure containing an absorber plate for absorbing the incident solar radiation, flow passages integrated with the absorber plate so that the fluid can be heated while flowing through the flow passages, and one or more covers (glazings) that trap the incident solar radiation and thereby increase the efficiency of heat recovery by the solar panel.
Thus the field of this invention is in the category of flat plate solar collectors for heating fluids.
Description of Prior Art
Most flat plate solar panels developed in earlier years used a variety of metals for the flow passages, absorber and frame, and either glass or polymer for the cover(s). For example, Duffie and Beckman (“Solar Engineering of Thermal Processes,” by John A. Duffie and William A. Beckman, Wiley Interscience Publication, John Wiley & Sons, New York 1980) describe typical flat plate collectors, and most use glass as cover(s) and copper, steel or aluminum for absorbers and tubing. U.S. Pat. No. 3,951,128 to Schoenfelder (1976) describes a flat plate-focal point collector that uses a reflector plate with mirror reflecting surface such as aluminum, steel or other suitable alloys, and dark painted conduits of copper. U.S. Pat. No. 4,011,856 to Gallager (1977) describes a flat plate collector that uses aluminum, base panel of Masonite, plywood or similar stiff, flat material and conduits of copper. More recently U.S. Pat. No. 5,974,611 to Bottum, Jr., et al (1998) describes a collector that uses copper tubes and plate with copper alloy fillets coupling tubes to plate.
While polymers have been used for many years for swimming pool heaters, such systems typically are for low temperature applications and most do not use a cover. In addition the polymers used in such applications are relatively low temperature materials that cannot withstand the temperatures required for heating of buildings and the heating of water for residential and industrial use. The solar panel assembly developed by Rhodes and Backlund (U.S. Pat. No. 4,205,662 dated Jun. 3, 1980 and assigned to Fafco, Inc) provides low temperature heating of water or other heat transfer fluids by use of various low-temperature polymers such as polyolefin, fiberglass reinforced polyester, polycarbonate, and acrylonitrile butadiene styrene (ABS). This panel provided for typical operating conditions for heating water up to 38 C (100 F) and for stagnation temperatures up to 93 C (200 F). Such panels have been used successfully for many years for swimming pool heaters. More recently (2009) Rhodes, et al (U.S. Pat. No. 8,161,963) developed a solar collector with an integrated storage tank that uses a thermosiphon effect to collect hot water within the storage tank. This panel uses thin film polyethylene sheets, acrylate, and polyvinyl chloride, all low temperature polymers. An integrated collector system using molded polymers is described in U.S. Pat. No. 6,814,070 (2004). This system combines heat collection and hot water storage in a single “passive” unit, and utilizes materials such as polycarbonate or acrylic, isocyanurate, cross-linked polyethylene (PEX), and aluminum.
U.S. Patent Publication No. 20130112190 describes a solar collector that utilizes polymer materials and consists of two manifolds plus a plurality of individual tubes for fluid flowing through the panel for collection of solar energy. Potential materials for the panel include high temperature nylon (HTN), chlorinated polyvinyl chloride (CPVC), polypropylene, polyethylene, polybutylene for the tubes, and polycarbonate, polyvinyl chloride (PVC), and polymethyl methacrylate (PMMA) for the cover, which is optionally also UV protected. The materials for the seals are rubber such as EPDM, VITON (etc), or a thermoplastic elastomer. The panel incorporates circular tubes with unique manifolds and sealing arrangements. The panel also includes optional compartments to enclose the individual tubes. The collector is designed so that one of the manifolds is rigidly fixed to the housing and the other manifold is flexibly fixed to the housing with flexible tubing in order to accommodate thermal expansion and contraction of the tubes. The tubes in the collector thermally expand lengthwise individually in and out of the enclosure past seals around the individual tubes. Such a design allows the use of either polymers or metals such as aluminum for the frame. The unit uses drainage holes to drain out of the enclosure any liquid or dirt that may have entered the enclosure.
U.S. Pat. No. 6,014,967 to Rekstad et al. (2000) describes an “Operating Method for Solar Collector Plant, and Control Unit for a Liquid Pump in a Solar Collector Plant.” The solar panel for this system utilizes a number of channels for conducting heat transfer fluid that accommodate particles for capillary effect to lift the fluid to effectively sweep the sun-facing surface layer of the absorber. The absorber unit in the solar collector will have air in the absorber after prolonged standstill, and an objective of the operating method is to provide for efficient startup of the system after standstill while avoiding unstable flow. The invention also relates to a control unit for the pump providing liquid circulation in the solar collector. The control unit utilizes a microprocessor or other computer or logic-calculating circuit along with a solar radiation sensor adjacent to the absorber and a temperature sensor in the heat storage medium to control the pump operation. Thus the primary focus of the invention is to effectively manage the startup and shutdown of the pump for a solar collector that accumulates air in the flow passages during standstill.